This application relates to optical fibers, and more specifically to apparatus and methodology for the efficient coupling of optical fibers in high power applications.
High power performance of fiber lasers and fiber amplifiers is increasingly being required for a wide variety of applications and operating optical powers have increased enormously in recent years. Components used in these products are generally based on passive micro-optical parts assembled by attaching optical fibers (polarization maintaining fibers or single mode fibers) to them through input and output lenses. Such components, originally designed for low power applications (typically in the mW range), are now required to operate at powers ranging from several watts to kW levels. These components are not designed for, and cannot reliably operate at, such high optical intensities. Many of the passive optical components used in these products require polished fiber ends, radiating into the air directly from the polished (or cleaved) fiber tip, through an anti-reflection (AR) coating directly deposited on the polished (or cleaved) fiber tip, or using an anti-reflection coated plate epoxied on the fiber tip (by way of example: a glass plate, previously anti-reflection coated, mounted to the end of the fiber with an index matching epoxy).
At these fiber tip junctions, the optical intensities are at their highest (reaching many GW/cm2), easily exceeding the maximum allowable damage levels of the bare fiber tip, the direct anti-reflection coating or anti-reflection plate bonding epoxy. Therefore, these points represent the weakest points in the transmission path and are likely to be damaged with the exposure to high optical power.
At present, there are two ways to reduce the optical intensity at the fiber tips in such situations. Firstly, by fusion splicing the optical fibers directly to the lenses, and secondly by means of locally expanding the fiber core by excessive localized thermal heating. The first solution has the drawback that the lens needs to match the index of the fiber perfectly (to reduce undesirable and problematic back reflections), resulting in limitations in choice of lens and lens performance. The fiber also needs to be placed exactly in the focus of the lens for optimum optical coupling, requiring tight tolerances in lens lengths and fiber to lens positioning. The process is also tedious and not cost effective.
The second solution has the drawback that the area over which the core is being expanded is relatively short, thus requiring great care when polishing or cleaving the fiber end, to avoid shortening the length over which the core is expanded. Shortening the length over which the core is expanded will reduce the size of the expanded beam, resulting in less than optimal reduction of the optical intensity. This process may also not be compatible with polarization maintaining fibers (PMF) because the severe thermal treatment will also deleteriously affect or destroy the internal stress originally induced and frozen into the polarization maintaining fiber thus severely reducing the polarization maintaining properties of the fiber. Furthermore, the fiber cores on both sides of the device need to be matched in mode size, and therefore also in the length over which the core is expanded, in order to optimize optical coupling.